This invention relates to a horizon sensor for providing pitch and roll attitude as well as altitude information of an orbiting body by scanning a line of discontinuity of optical radiation in a scan field of view and more particularly, in such a sensor having a scanner which simultaneously scans two concentric conical paths through the field of view about the same axis of rotation using the same scanner and detector to provide two axes as well as altitude information.
Horizon sensors are devices mounted in orbiting bodies such as satellites, missles, space probes, extremely high flying aircraft and the like which sense the horizon of a reference body and produce a signal which indicates orientation of the orbiting body with respect thereto. The attitude of the orbiting body is determined by its position with respect to three axes at right angles to each other, two of which are in a plane at right angles to a projected radius of the earth passing through the orbiting body and the third coincides with such radius. The plane in which the first two axes lie is parallel to the earth's horizon and are referred to as the pitch and roll axes with which the present invention deals.
In U.S. Pat. No. 3,020,407 which is assigned to the Assignee of the present invention, a horizon sensor is provided which utilizes the horizon representing a line of discontinuity between the earth's atmosphere and outer space which is characterized by large differences in infrared radiation. By scanning an infrared detector in a conical scan pattern across the horizon and deriving electrical output signals marking the line of discontinuity between cold outer space and the warm earth provides an electrical output from the detector with a large increase in signal as the line of thermal discontinuity is crossed. By generating reference pulses as the scan passes predetermined points in the orbit of the orbiting body the intervals between horizon crossings and the reference pulses may be used to produce an output signal providing attitude information of the orbiting body with respect to the horizon.
The optical configuration of a typical conical scan horizon sensor is shown in FIG. 1 in which an infrared detector views a small instantaneous field in space on the order of 1.degree. diameter which scans through a large circle on a celestial sphere.
FIG. 2 shows the path of the sensor of FIG. 1 scanning across the earth along line AB having an instantaneous field of view crossing on and off the earth at points A and B, respectively. The detector in such a scan will produce an approximately rectangular wave shaped signal as shown in FIG. 3 having a width AB. The pitch and roll axis of the orbiting body or space craft are defined in FIG. 2. The pitch attitude of the space craft can be determined by the phase .phi. of the midpoint of the rectangular wave with respect to an internal reference position R. The roll attitude can be determined by the difference in the width of the rectangular wave shape AB and that generated by another conical scanner, CD displaced toward the opposite side of the earth. The problem of course, resides in the fact that two conical scanner sensor heads are required to provide this two axis information, which increases the weight, power and cost of the system as well as providing more moving parts and increasing the chance of malfunction.